DISPERSAL DURING GLACIAL PERIOD 411 



neighbouring island, but not from one distant continent to 

 another. The floras of distant continents would not by such 

 means become mingled ; but would remain as distinct as they 

 now are. The currents, from their course, would never 

 bring seeds from North America to Britain, though they 

 might and do bring seeds from the West Indies to our west- 

 ern shores, where, if not killed by their long immersion in 

 salt-water, they could not endure our climate. Almost every 

 year, one or two land-birds are blown across the whole At- 

 lantic Ocean, from North America to the western shores of 

 Ireland and England; but seeds could be transported by these 

 rare wanderers only by one means, namely, by dirt adhering 

 to their feet or beaks, which is in itself a rare accident. 

 Even in this case, how small would be the chance of a seed 

 falling on favourable soil, and coming to maturity ! But it 

 would be a great error to argue that because a well-stocked 

 island, like Great Britain, has not, as far as is known (and 

 it would be very difficult to prove this), received within the 

 last few centuries, through occasional means of transport, 

 immigrants from Europe or any other continent, that a 

 poorly-stocked island, though standing more remote from the 

 mainland, would not receive colonists by similar means. Out 

 of a hundred kinds of seeds or animals transported to an 

 island, even if far less well-stocked than Britain, perhaps 

 not more than one would be so well fitted to its new home, 

 as to become naturalised. But this is no valid argument 

 against what would be effected by occasional means of trans- 

 port, during the long lapse of geological time, whilst the 

 island was being upheaved, and before it had become fully 

 stocked with inhabitants. On almost bare land, with few or 

 no destructive insects or birds living there, nearly every seed 

 which chanced to arrive, if fitted for the climate, would ger- 

 minate and survive. 



DISPERSAL DURING THE GLACIAL PERIOD 



The identity of many plants and animals, on mountain- 

 summits, separated from each other by hundreds of miles of 

 lowlands, where Alpine species could not possibly exist, is 

 one of the most striking cases known of the same species 



